a cell are "on," or "expressed," and 
giving orders for the production of 
specific proteins. Some of the instruc- 
tions that switch these genes on or off 
come from the cytoplasm, where they 
are generated as a result of interac- 
tions between the surface membrane 
and the environment. Thus, the com- 
mands from the nucleus are influenced 
by what goes on outside the cell as 
well as by the cell's genetic program. 
An order to make a protein begins 
when the appropriate genes are "tran- 
scribed" from the DNA into strands of 
another kind of nucleic acid, called 
messenger ribonucleic acid (mRNA). 
Messenger RNA is manufactured by 
transcribing just one chain of the DNA 
double helix (one side of the twisted 
ladder). A strand of mRNA is comple- 
mentary to the DNA from which it is 
transcribed, except that each adenine 
of the DNA is paired with a uracil 
(U), instead of with thymine. For ex- 
ample, a stretch of DNA bases ATCG 
is transcribed into the mRNA se- 
quence UAGC. 
After additional processing, the 
mRNA carries its message into the 
cytoplasm, while the DNA remains 
safely in the nucleus, somewhat like 
the printing block in a printing press. 
It may be that the mRNA gets out 
through "gates" in the nuclear mem- 
brane called nuclear pore complexes, 
although this theory is still unproved. 
Once in the cytoplasm, the messen- 
ger RNA carries its instructions to tiny 
organelles called ribosomes, the 
"factories" in which the next step of 
protein manufacture, called transla- 
tion, takes place. 
24 
